Diesel-fueled internal combustion engines are widely used to power vehicles such as medium and heavy duty trucks and have other applications as in emergency generators. Whereas gasoline engines use spark ignition, diesel engines use compression ignition. As a consequence, the composition of diesel engine exhaust is much different from that of gasoline engine exhaust. The major pollutants in gasoline engine exhaust are carbon monoxide, unburned hydrocarbons, and some NOx. The major pollutants in diesel engine exhaust are NOX and particulate matter (soot). Gasoline engine exhaust aftertreatment systems have been widely used since the 1970s. Diesel engine exhaust aftertreatment systems have only recently come into widespread use.
Catalytic converters comprising three-way catalysts can effectively control emissions from conventional gasoline engines by oxidizing carbon monoxide and unburned hydrocarbons while simultaneously reducing NOX. This approach is unsuitable for diesel engine exhaust because diesel exhaust contains from about 4 to 20% oxygen. The excess of oxygen and dearth of oxygen accepting species (reductants) makes catalytic converters ineffective for reducing NOX in diesel exhaust.
Several solutions have been proposed for controlling NOX emissions from diesel engines. One set of approaches focuses on the engine itself. Techniques such as exhaust gas recirculation and partially homogenizing fuel-air mixtures are helpful, but these techniques alone do not eliminate NOX emissions. Another set of approaches remove NOX from the exhaust. These include the use of lean-burn NOX catalysts, selective catalytic reduction (SCR) catalysts, and lean NOX traps (LNTs).
Lean-burn NOX catalysts promote the reduction of NOx under oxygen-rich conditions. Reduction of NOX in an oxidizing atmosphere is difficult. It has proven challenging to find a lean-burn NOx catalyst that has the required activity, durability, and operating temperature range. A reductant such as diesel fuel must be steadily supplied to the exhaust for lean NOX reduction, adding 3% or more to the engine's fuel requirement. Currently, the sustainable NOX conversion efficiencies provided by lean-burn NOX catalysts are unacceptably low.
SCR generally refers to selective catalytic reduction of NOX by ammonia. The reaction takes place even in an oxidizing environment. The NH3 or NOX can be temporarily trapped and stored in an adsorbent with an exhaust system, or ammonia can be fed continuously to the exhaust. SCR can achieve high levels of NOX reduction, but there is a disadvantage in the lack of infrastructure for distributing ammonia or a suitable precursor. Another concern relates to the possible release of ammonia into the environment.
LNTs are devices that adsorb NOX under lean conditions and reduce and release the adsorbed NOX under rich conditions. An LNT generally includes a NOX adsorbent and a catalyst. The adsorbent is typically an alkali or alkaline earth compound, such as BaCO3 and the catalyst is typically a combination of precious metals including Pt and Rh. In lean exhaust (exhaust containing an excess of oxygen and other oxidizing species in comparison to reducing compounds), the catalyst speeds reactions that lead to NOX adsorption. In a rich exhaust (containing reductants in excess of oxidizing compounds), the catalyst speeds reactions by which reductants are consumed and adsorbed NOX is reduced and desorbed. In a typical operating protocol, a rich condition (reducing environment) is created within the exhaust from time-to-time to regenerate (denitrate) the LNT.
In addition to accumulating NOX, LNTs accumulate SOX. Most fuels contain some sulfur and SOX is a byproduct of combusting sulfur-containing fuels. Even with low sulfur diesel fuels, the amount of SOX produced by combustion is significant. SOX adsorbs more strongly than NOX and necessitates a more stringent, though less frequent, regeneration under rich conditions. Desulfation requires elevated temperatures, e.g., 700° C.
The reducing environment for denitration or desulfation can be created in a variety of ways. One approach uses the engine to create a rich exhaust-reductant mixture. For example, the engine can inject extra fuel into the exhaust within one or more cylinders prior to expelling the exhaust. A disadvantage of this approach is that it can interfere with normal engine operation. Another approach is to inject reductant into the exhaust downstream from the engine.
With either approach a portion of the reductant is expended consuming excess oxygen in the exhaust. Rerouting a portion of the exhaust to bypass the LNT during regeneration reduces the required amount, but it is preferable to avoid the use of bypass lines and exhaust valves. A more acceptable approach to reducing the amount of excess oxygen and the amount of reductant required to reduce it is throttling the engine, at least under circumstances where such throttling does not have an adverse effect on engine performance.
Reductant reacts with excess oxygen in the exhaust to form combustion or partial combustion products. Preferably, the reactions take place upstream from the LNT in a fuel reformer. A fuel reformer is a device that catalyzes oxidation reactions in such a way as to favor the formation of partial combustion reaction products, especially CO and H2. CO and H2 are more effective than their precursors for regenerating LNTs. Fuel reformers are sometime placed outside of the exhaust line to provide reductant in the form of CO and H2. It is preferable, however, for the fuel reformer to be configured within the exhaust line and to generate CO and H2 while simultaneously eliminating excess oxygen from the exhaust.
WO 2004/090296 (hereinafter “the 296 publication”) describes a diesel automotive exhaust treatment system with a fuel reformer configured within an exhaust line upstream from LNT and SCR catalysts. The reformer removes excess oxygen from the exhaust while converting diesel fuel into more reactive reformate. In that system, the fuel reformer is combined into a diesel particulate filter. The combination reformer-particulate filter has a high thermal mass and must perform its reforming function at exhaust gas temperatures. This requires a relatively large amount of precious metal catalyst. The combination fuel reformer-particulate filter is heated over an extended time interval on a periodic basis in order to remove accumulated soot.
U.S. Pat. Pub. No. 2004/0050037 (hereinafter “the '037 publication”) describes a different type of fuel reformer for placement in an exhaust line upstream from an LNT. The '037 reformer includes both oxidation and steam reforming catalysts. Pt and/or Pd serves as the oxidation catalyst. Rh serves as the reforming catalyst. The inline reformer of the '037 publication is designed for rapid heating and then catalyzing steam reforming. Temperatures from about 500° C. to about 700° C. are required for effective function of this reformer. These temperatures are substantially higher than typical diesel exhaust temperatures. The reformer is heated by injecting fuel at a rate that leaves the exhaust lean, whereby the injected fuel combusts to generate heat. After warm up, the fuel injection rate is increased and or the oxygen flow rate reduced to provide a rich exhaust.
Designing the fuel reformer to heat and operate at least partially through steam reforming reactions as in the '037 publication as opposed to operating at typical diesel exhaust temperatures as in the '296 publication reduces the catalyst requirement, increases reformate (CO and H2) yield, and reduces the amount of waste heat. In principal, if reformate production proceeds through partial oxidation reforming as in the reaction:CH1.85+0.5O2→CO+0.925H2  (1)1.925 moles of reformate (moles CO plus moles H2) could be obtained from each mole of carbon atoms in the fuel. CH1.85 is used to represent diesel fuel having a typical carbon to hydrogen ratio. If reformate production proceeds through steam reforming as in the reaction:CH1.85+H2O→CO+1.925H2  (2)2.925 moles of reformate (moles CO plus moles H2) could in principle be obtained from each mole of carbon atoms in the fuel. In practice, yields are lower than theoretical amounts due to the limited efficiency of conversion of fuel, the limited selectivity for reforming reactions over complete combustion reactions, the necessity of producing heat to drive steam reforming, and the loss of energy required to heat the exhaust. Nevertheless, the benefits are sufficient that a low thermal mass reformer that must be preheated is preferred over a large thermal mass reformer that does not require preheating. The fuel expended preheating the fuel reformer is more than compensated by the benefits of steam reforming.
Another advantage of a fuel reformer that heats to steam reforming temperatures for each LNT denitration is that such a reformer is less susceptible to soot build-up. Soot is not only present in the exhaust, but is also a byproduct of fuel reforming. When the fuel reformer is heated, the soot can burn away. This burning can take place during the lean warm-up phase or after the completion of regeneration. Some soot may also be removed by gasification reactions under rich conditions. Soot accumulation can still be a problem, however, if the fuel reformer has cool spots.
The formation of cool spots is a function of the catalysts structure. The structure of a fuel reformer is typically a monolith. Monolith catalysts have become the standard in exhaust aftertreatment. In the early 1970s, when catalytic converters were first introduced by the automobile industry, the catalyst was supported on pellets that were packed into containers. With these packed beds, it was difficult to keep back pressure within engine tolerances. Degradation of the pellets due to vibrations compounded the problem. A pancake shape having a broad frontal area and a shallow bed depth was adapted to reduce back pressure. This shape was difficult to fit under vehicles and it was difficult to maintain a uniform flow across the enlarged cross-section.
Monolith catalysts replaced packed beds for exhaust aftertreatment systems. In a monolith catalyst, the catalytic material is disposed on surfaces in a structured array of longitudinally oriented channels. A honeycomb is a typical monolith structure. The structured orientation of the channels results in a reduced pressure drop in comparison with a packed bed for a given degree of exhaust-catalyst contacting and a given frontal area. The cohesive monolith structure resists degradation better than a packed bed.
Monolith catalyst substrates are generally either ceramic or metallic. Ceramic monoliths can be extruded. Metallic monoliths are formed from metal foils. Typically, at least one of the foils is textured, for example corrugated. Through various combinations of stacking, folding, and/or rolling, the foils can be formed into a monolith structure. FIGS. 1 and 2 provide an example in which a flat foil 1 and a corrugated foil 2 are laid together than rolled to form the monolith 3, which is shown in cross-section.
Monolith-supported exhaust system catalysts have well defined heat and mass transport characteristics. When the catalyst is hot and very active, reaction rates are limited by transport of chemical species from the laminar flow within the channels to the surfaces of the catalyst disposed at the walls of the channels. Mass transport is by diffusion through the relatively stagnant exhaust layers adjacent the channel walls. A single dimensionless coefficient, the Nusselt number, characterizes the transport rates.
Channel densities for monoliths used in exhaust systems are typically in the range from 1×101 to 2×102 per cm2. Packaging consideration place an upper limit on the frontal area of the monolith. The frontal area, which is generally the cross-sectional area for the monolith at any point along its length, is typically in the range from 1×101 to 1×103 per cm2. These parameters assure that flow within the channels is well within the laminar range.
Within a short distance of the entrance of a monolith, the flow profile and the Nusselt number approach asymptotic limits corresponding to fully developed laminar flow. In the limit of fully developed laminar flow, the Nusselt number depends only on channel shape. In the entrance region, where the laminar flow profile is still developing, the Nusselt number can be much higher than the asymptotic limit, which means that transport rates are enhanced.
U.S. Pat. Nos. 5,045,403 (hereinafter the '043 patent) and 6,316,121 (hereinafter the '121 patent) describe modified monolith structures that take advantage of the enhanced Nusselt numbers of a developing flow. These patents described monoliths channels that are modified by secondary structures. FIG. 4 provides an example from the '121 patent. The structure 15 has a primary structure 14, which defines flow channels. Secondary structures 11, the shapes of which are most easily appreciated at their ends 12, intrude on the channels defined by the primary structure, causing the flow to change shape and redevelop periodically through the monolith length. FIG. 3 provides an exemplary structure 25 from the '403 patent. Corrugations 26 provide the primary structure. Secondary structures in the form of peaks 27 and valleys 28 intrude into the channels formed by the primary structure. These secondary structures provide internal leading edges 29. All secondary structure provide locally enhanced Nusselt numbers, but internal leading edges force particularly sudden and sharp changes to the flow profile and particularly high Nusselt numbers. In each case, the secondary structures redirect the exhaust flow periodically through the monolith lengths, creating entrance type conditions through the monolith interiors.
Nusselt numbers are made dimensionless using a ratio of parameters, one of which is a local transport rate coefficient and another of which characterizes the channel size. Reducing channel sizes does not alter the Nusselt numbers, but provides another way to increase transport rates. Reducing channels sizes generally involves increasing the number of channels so as to maintain the frontal area and also decreasing the monolith length to keep the total surface area and catalyst amount the same. In spite of shorter length, reduced channel sizes increase back pressure. Secondary structures like internal leading edges also increases back pressure. In both cases increasing mass transport rates comes at the cost of increasing back pressure. It is unclear whether introducing the internal leading edges provides a better tradeoff than reducing channel sizes. Manufacturing considerations and material limits may ultimately drive choices.
In spite of advances, there continues to be a long felt need for an affordable and reliable diesel exhaust aftertreatment system that is durable, has a manageable operating cost (including fuel requirement), and reduces NOX emissions to a satisfactory extent in the sense of meeting U.S. Environmental Protection Agency (EPA) regulations effective in 2010 and other such regulations that limit NOX emissions from trucks and other diesel-powered vehicles.